Methods and apparatus for synchronizing rf pulses in a plasma processing system

ABSTRACT

A synchronized pulsing arrangement for providing at least two synchronized pulsing RF signals to a plasma processing chamber of a plasma processing system is provided. The arrangement includes a first RF generator for providing a first RF signal. The first RF signal is provided to the plasma processing chamber to energize plasma therein, the first RF signal representing a pulsing RF signal. The arrangement also includes a second RF generator for providing a second RF signal to the plasma processing chamber. The second RF generator has a sensor subsystem for detecting values of at least one parameter associated with the plasma processing chamber that reflects whether the first RF signal is pulsed high or pulsed low and a pulse controlling subsystem for pulsing the second RF signal responsive to the detecting the values of at least one parameter.

PRIORITY CLAIM

This application is a divisional of and claims the benefit of andpriority under 35 U.S.C. §120, to U.S. patent application Ser. No.13/550,719, filed on Jul. 17, 2012, and titled “METHODS AND APPARATUSFOR SYNCHRONIZING RF PULSES IN A PLASMA PROCESSING SYSTEM”, which claimspriority under 35 USC. §119(e) to a provisional Patent Application No.61/602,041, filed on Feb. 22, 2012, and titled “METHODS AND APPARATUSFOR SYNCHRONIZING RF PULSES IN A PLASMA PROCESSING SYSTEM”, all of whichare incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

Plasma processing has long been employed to process substrates (e.g.,wafer or flat panels or other substrates) to create electronic devices(e.g., integrated circuits or flat panel displays). In plasmaprocessing, a substrate is disposed in a plasma processing chamber,which employs one or more electrodes to excite a source gas (which maybe an etchant source gas or a deposition source gas) into a plasma forprocessing the substrate. The electrodes may be excited by an RF signal,which is furnished by a RF generator, for example.

In some plasma processing systems, multiple RF signals, some of whichmay have the same or different RF frequencies, may be provided to one ormore electrodes to generate plasma. In a capacitively-coupled plasmaprocessing system, for example, one or more RF signals may be providedto the top electrode, the bottom electrode, or both in order to generatethe desired plasma.

In some applications, the RF signals may be pulsed. For any given RFsignal, RF pulsing involves turning the RF signal on and off, typicallywithin the same RF signal period but may span multiple RF signalperiods. Furthermore, the RF pulsing may be synchronized among signals.For example, if two signals RF1 and RF2 are synchronized, there is anactive pulse of signal RF1 for every active pulse of signal RF2. Thepulses of the two RF signals may be in phase, or the leading edge of oneRF pulse may lag behind the leading edge of the other RF pulse, or thetrailing edge of one RF pulse may lag behind the trailing edge of theother RF pulse, or the RF pulses may be out of phase.

In the prior art, pulsing synchronization of multiple RF signalstypically involves a communication network to facilitate controlcommunication among the various RF generators. To facilitate discussion,FIG. 1 is a high level drawing of a generic prior art implementation ofa typical pulsed RF plasma processing system 102. Pulsed RF plasmaprocessing system 102 includes two RF generators 104 and 106. In theexample of FIG. 1, RF generator 104 represents a 2 MHz generator whileRF generator 106 represents a 60 MHz generator.

A host computer 110 implements tool control and receives a feedbacksignal 112 from an impedance matching network 114 to provide (via adigital or analog communications interface 116) power set point data toRF generator 104 and RF generator 106 via paths 118 and 120respectively. The feedback signal 112 pertains to the impedance mismatchbetween the source and the load, and is employed to control either thedelivered power or the forward power levels of RF generators 104 and 106to maximize power delivery and minimize the reflected power.

Host computer 110 also provides Pulse_Enable signal 160 to a pulsesynchronizer and controller 130. Responsive to the Pulse_Enable signal160, the pulse synchronizer and controller 130 provides the synchronizedcontrol signals 170 and 172 to RF generator 104 and RF generator 106(via External Synchronization Interfaces 140 and 142) to instruct RFgenerators 104 and 106 to pulse its RF signals using power controllers150 and 152 respectively to produce pulsed RF signals 162 and 164. Thepulsed RF signals 162 and 164 are then delivered to the load in plasmachamber 161 via impedance matching network 114.

Although the pulsed RF synchronization scheme of FIG. 1 can provide thesynchronized pulsing function for the RF generators, there aredrawbacks. For example, synchronizing the pulsing function of thevarious RF generators in FIG. 1 requires the use of a network tocommunicate among host computer 110, pulse synchronizer/controller 130,and external synchronization interfaces 140 and 142 in RF generators 104and 106. Further, synchronizing the pulsing function of the various RFgenerators in FIG. 1 requires the implementation of the externalsynchronization interfaces (such as 140 and 142) in the variousgenerators. Implementing these external synchronization interfaces addsan extra layer of complexity to RF generator designs, and renderexisting RF generators incapable of being used for RF synchronizedpulsing.

In view of the foregoing, there are desired improved techniques andsystems for implementing synchronized RF pulsing in a plasma processingsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a high level drawing of a generic prior art implementation ofa typical pulsed RF plasma processing system.

FIG. 2 shows a timing diagram of the pulsing of a 2 MHz RF signal toillustrate the change in gamma value for one RF generator when anotherRF generator pulses its RF signal.

FIG. 3 shows a simplified circuit block diagram of an implementation ofthe synchronized pulsing RF, in accordance with an embodiment of theinvention.

FIG. 4 is an example implementation of a DP RF generator for providingthe synchronized RF pulsing capability, in accordance with an embodimentof the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention will now be described in detail with reference toa few embodiments thereof as illustrated in the accompanying drawings.In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one skilled in the art, that the presentinvention may be practiced without some or all of these specificdetails. In other instances, well known process steps and/or structureshave not been described in detail in order to not unnecessarily obscurethe present invention.

Various embodiments are described herein below, including methods andtechniques. It should be kept in mind that the invention might alsocover articles of manufacture that includes a computer readable mediumon which computer-readable instructions for carrying out embodiments ofthe inventive technique are stored. The computer readable medium mayinclude, for example, semiconductor, magnetic, opto-magnetic, optical,or other forms of computer readable medium for storing computer readablecode. Further, the invention may also cover apparatuses for practicingembodiments of the invention. Such apparatus may include circuits,dedicated and/or programmable, to carry out tasks pertaining toembodiments of the invention. Examples of such apparatus include ageneral-purpose computer and/or a dedicated computing device whenappropriately programmed and may include a combination of acomputer/computing device and dedicated/programmable circuits adaptedfor the various tasks pertaining to embodiments of the invention.

Embodiments of the invention relate to methods and apparatus forimplementing synchronized pulsing of RF signals in a plasma processingsystem having a plurality of RF generators. In one or more embodiments,one of the RF generators is designated the independent pulsing (IP) RFgenerator, and other RF generators are designated dependent pulsing (DP)generators.

The IP RF generator represents the RF generator that pulsesindependently from the DP RF generators. The IP RF generator(independent pulsing generator) generates its RF pulses responsive to asignal from the tool host or another controller. The DP RF generators(dependent pulsing generators) monitor the change in plasma impedancethat is characteristic of pulsing by the IP RF generator and triggertheir individual RF pulses responsive to the detected change in plasmaimpedance. In one or more embodiments, the change in the plasmaimpedance is detected by the power sensor in each of the DP RFgenerators, which may measure, for example, the forward and reflected RFpowers.

The inventors herein recognize that existing RF generators are alreadyprovided with sensors (such as power sensors) which can monitorparameters related to the plasma impedance. When the values of theseparameters change in a certain manner, a change in the plasma impedancemay be detected.

To further elaborate, the efficiency with which an RF generator deliversRF power to a load depends on how well the load impedance matches withthe source impedance. The more closely the load impedance matches thesource impedance, the more efficient the RF power is delivered by an RFgenerator. Since this matching issue is well-known, many or most priorart RF generators have been provided with the ability to sense themismatch between the source impedance and the load impedance, and toadjust the delivered or forward power in order to reduce the mismatch.The parameter gamma is typically employed to measure the load-sourceimpedance mismatch. A gamma value of zero indicates perfect matchingwhile a gamma value of 1 indicates a high degree of mismatch. In some RFgenerators, this gamma value is calculated from values provided by thepower sensor, which detects the source and reflected RF powers.

The inventors herein further realize that the plasma impedance is afunction of power delivered to the plasma. When a given RF generator(referred to herein as the independent pulsing or IP RF generator)pulses, the delivered RF power changes, and the plasma impedance changesaccordingly. Other RF generators (referred to herein as dependentpulsing or DP RF generators) react to this change in the plasmaimpedance by varying their power output to match their source impedancewith the plasma (or load) impedance.

The detection of changes in the plasma impedance typically relies on themeasurement of one or more parameters whose values can be analyzed todirectly or indirectly ascertain changes in the plasma impedance. If theplasma impedance change caused by RF pulsing of the IP RF generator canbe detected by other RF generators, and more importantly, if thisdetection can be used to trigger RF pulsing by these other RFgenerators, synchronized pulsing can be achieved without the need toexplicitly link the RF generators via a control network as is done inthe prior art.

To illustrate the change in gamma value for one RF generator whenanother RF generator pulses its RF signal, FIG. 2 shows a timing diagramof the pulsing of a 2 MHz RF signal 202, which is pulsed at 159 Hz, witha 50% duty cycle. In the example of FIG. 2, two RF generators areinvolved: a 2 MHz RF generator outputting 6000 Watts RF signal and a 60MHz RF generator outputting a 900 Watts RF signal. The 2 MHz RF signalis pulsed between 6000 Watts and 0 Watts, as discussed, while the 60 MHzRF signal (204) is not pulsed.

When the 2 MHz RF signal 202 is active (from reference number 210 to212), the RF power sensor of the 60 MHz RF generator reacts to theplasma impedance value caused by the high 2 MHz RF signal 202. In thiscase, the real value of the impedance at the match input (generatoroutput) of the 60 MHz RF generator is 52.9 ohms. The gamma value, whichdescribes the source-load impedance mismatch, is 0.039.

When the 2 MHz RF signal 202 is inactive (from reference number 212 to214), the RF power sensor of the 60 MHz RF generator reacts to theplasma impedance caused by the low 2 MHz RF signal 202. In this case,the real value of the impedance at the match input (generator output) ofthe 60 MHz RF generator is only 27.44 ohms. The gamma value, whichdescribes the source-load impedance mismatch, is 0.856.

As can be seen in the example of FIG. 2, either the impedance at thematch input or the gamma value may be monitored and if a change occursfrom the value reflective of the “on” state of the 2 MHz RF signal 202to the value reflective of the “off” state of the 2 MHz RF signal 202(or vice versa), the detection of such change may be employed as atrigger signal to a circuit to generate an RF pulse for the 60 MHzsignal of the 60 MHz DP RF generator. If there are other DP RFgenerators, each DP RF generator may monitor the plasma impedance (e.g.,a parameter that is directly or indirectly reflective of this plasmaimpedance) and use the detection of plasma impedance change to triggerpulse generation. In this manner, no explicit control network between amaster control circuit/device (such as from host computer 110 or pulsesynchronization controller circuit 130) and the various RF generators isneeded. Further, the RF generators do not require any additionalcircuitry to interface with the control network (such as externalsynchronization interface circuits 140 and 142 of FIG. 1).

Instead, only one RF generator (the IP RF generator such as the 2 MHz IPRF generator in the example) needs to be explicitly controlled for RFpulsing. Other RF generators (the DP RF generators) leverage on existingdetection circuitry (which is traditionally used to monitor the forwardand reflected RF power for adjusting the power set point for RF deliveryto match the source impedance to the load impedance) in order toindirectly detect when the IP generator RF signal has pulsed. Thisdetection provides a triggering signal to the DP RF generators to allowthe DP RF generators to generate their own RF pulses in response to thedetection of RF pulsing by the IP RF generator. In this manner, vastlymore simplified synchronized pulsing is accomplished.

The features and advantages of embodiments of the invention may bebetter understood with reference to the figures and discussions thatfollow. FIG. 3 shows a simplified circuit block diagram of animplementation of the synchronized pulsing RF 300, in accordance with anembodiment of the invention. In FIG. 3, RF generator 302 represents theIP RF generator and receives its pulsing control signal from tool hostcomputer 304 (via digital/analog communications interface 306). IP RFgenerator 302 then generates, using power controller 308, an RF pulseusing a power setpoint provided by tool host computer 304. The pulse isfurnished to impedance matching network 314 to energize the RF-drivenplasma chamber 316. The plasma impedance in RF-driven plasma chamber 316changes as a result of the on-state of the 2 MHz pulse from IP RFgenerator 302.

This plasma impedance change is then detected by RF sensor 320 of DP RFgenerator 322. By way of example, the forward and reflected power of theDP 60 MHZ RF generator 322 may be monitored. Generally anIP_RF_Pulse_High threshold value may be employed to determine when the 2MHz pulse from the IP RF generator 302 is deemed to be high. In anembodiment, the gamma value obtained from measurements taken by RFsensor 320 is employed and compared against the aforementionedIP_RF_Pulse_High value. Once the 2 MHz pulse from the IP RF generator302 is deemed to be on, pulse generation circuit associated with DP RFgenerator 322 may be employed to generate a pulse for the 60 MHz signalfrom DP RF generator 322.

The pulse from DP RF generator 322 may be set to stay on for apredefined duration (e.g., in accordance with some duty cyclespecification) or may be synchronized to turn off when the 2 MHz pulsefrom IP RF generator 302 transitions from a high state to a low state(by monitoring the plasma impedance state in the manner discussedearlier).

FIG. 4 is an example implementation of a DP RF generator 400 forproviding the synchronized RF pulsing capability. In FIG. 4, a signal402 is provided from the tool host, which signal may include twoadditional values: a trigger threshold and a gain value. The triggerthreshold represents the predefined value for triggering the RF pulsefor the DP generator (which keys off the plasma impedance change causedby the independent pulsing generator). By way of example, if the gammavalue is monitored by the DP RF generator for detecting the plasmaimpedance change due to the pulsing of the IP RF generator, thethreshold value may represent the gamma value which, when traversed,represents the triggering signal for triggering the RF pulse by the DPRF generator. The gain value represents a value for scaling the signalto provide the high level and the low level of the RF pulse by the DP RFgenerator (since it is possible that different power levels may bedesired for high and low instead of full-on or full-off).

Returning now to FIG. 4, if the IDPC input is zero (block 404,signifying that the chamber is not operating in the RF pulsing mode),the RF pulsing functionality is bypassed in the example of FIG. 4. Inthis case, the default power set point (normally furnished by the toolhost computer to govern the power output by the RF generator) is sent tothe power amplifier (block 406) and amplified via the RF power amplifier408, which is then output to the plasma chamber 450 via path 410.

The RF sensors 412 monitors the forward and reflected powers in theexample of FIG. 4, and provides these values to logic circuit 414 inorder to permit default scaling circuit 416 to scale the power set pointto optimize power delivery. For example, if the gamma value is too high(indicating a large mismatch between the forward and reflected power),the power set point provided by the tool host may be increased ordecreased as necessary to optimize power delivery to the plasma load.

However, if the IDPC input is not equal to zero (block 404, signifyingthat the chamber is operating in the RF pulsing mode), the RF pulsingfunctionality is enabled in the example of FIG. 4 (via pulse powerscaling circuit 420). In this case, the power set point (furnished bythe tool host computer to govern the power output by the RF generatorand is part of the IDPC input in this case) is sent to the pulse powerscaling circuit 420. The scaling may toggle between two values, high andlow, depending on the detection of the plasma impedance by RF sensor 412and logic circuit 414.

Suppose RF sensor 412 and logic circuit 414 detect that the gamma valuehas traversed the trigger threshold value provided with signal 402, thisinformation is provided to pulse power scaling circuit 420, which thenscales the default power set point scaling to reflect the high RF pulsestate. Once pulse scaling is complete (block 420), the newly scaledpower setpoint is then sent to block 408 for RF amplification (via block406) and the high RF pulse level is sent to the plasma chamber. Toimplement a low pulse, another scaling value may be employed by block420 (e.g., upon detection of the low pulse of the IP RF generator orafter a predefined duration of time has past since the DP RF pulse wenthigh) to generate a low RF pulse level to be sent to the plasma chamber.

In an embodiment, a generalized method for synchronizing RF pulsing mayinvolve independent pulsing at least one RF power supply (the IP RFpower supply). Each of the other RF supplies may then monitor forindicia of plasma impedance change (such as gamma value, forward power,reflected power, VI probe measurement, real and/or complex values of thegenerator output impedance, etc.). In other words, detection that theplasma impedance has changed in a manner that is characteristic ofpulsing by the independent pulsing RF generator is not limited to gammamonitoring.

In an advantageous example, the DP RF generators may analyze VI probemeasurements and/or phase information received from the chamber in orderto detect plasma impedance change that is characteristic of pulsing bythe independent pulsing RF generator. Upon detection that the plasmaimpedance has changed in a manner that is characteristic of pulsing bythe independent pulsing RF generator (e.g., from low to high or high tolow), the dependent RF power supply may use that detection as a triggerto generate its pulse. The high RF pulse of the dependent RF generatormay persist for a predefined period of time, or the RF pulse of thedependent RF generator may transition to a low value upon detecting thatthe independent pulsing RF signal has transitioned to a low state.

As can be appreciated from the foregoing, embodiments of the inventiondetects plasma impedance change that is characteristic of pulsing eventsby the independent pulsing RF generator and employs the detection as atrigger signal to pulse the dependent pulsing RF generator. In thismanner, complicated networks and interfaces are no longer necessary tosynchronize pulsing among a plurality of RF generators.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents, whichfall within the scope of this invention. Although various examples areprovided herein, it is intended that these examples be illustrative andnot limiting with respect to the invention.

Also, the title and summary are provided herein for convenience andshould not be used to construe the scope of the claims herein. Further,the abstract is written in a highly abbreviated form and is providedherein for convenience and thus should not be employed to construe orlimit the overall invention, which is expressed in the claims. If theterm “set” is employed herein, such term is intended to have itscommonly understood mathematical meaning to cover zero, one, or morethan one member. It should also be noted that there are many alternativeways of implementing the methods and apparatuses of the presentinvention. It is therefore intended that the following appended claimsbe interpreted as including all such alterations, permutations, andequivalents as fall within the true spirit and scope of the presentinvention.

What is claimed is:
 1. A method comprising: generating a first radiofrequency (RF) signal, wherein the first RF signal is a pulse signal;sending the first RF signal to an impedance match network, wherein theimpedance match network is coupled to a plasma chamber; generating asecond RF signal; sending the second RF signal to the impedance matchnetwork; sensing a parameter indicating a change in an impedance ofplasma within the plasma chamber, wherein the change in the impedanceoccurs when a state of the first RF signal changes from one state toanother; and pulsing the second RF signal from one state to another uponsensing the parameter indicating the change in the impedance.
 2. Themethod of claim 1, wherein the parameter is sensed at an output of an RFgenerator that generates the second RF signal.
 3. The method of claim 1,wherein the first RF signal is a low-frequency RF signal and the secondRF signal is a high-frequency RF signal, wherein the high-frequency isgreater than the low-frequency, wherein the parameter is associated withforward power and reflected power.
 4. The method of claim 1, wherein theone state of the first RF signal is an on state and the other state ofthe first RF signal is an off state, wherein the one state of the secondRF signal is an on state and the other state of the second RF signal isan off state, wherein the change in the plasma impedance occurs as aconsequence of a transition of the first RF signal from the one state tothe other state, wherein the pulse of the first RF signal is generatedupon receiving a control signal from a computer, wherein the pulsing ofthe second RF signal is performed to synchronize the pulsing of thesecond RF signal with pulsing of the first RF signal.
 5. The method ofclaim 1, wherein the second RF signal is pulsed from the one state tothe other state when a predetermined power set point is applied to thesecond RF signal, wherein the second RF signal is pulsed from the otherstate to the one state when another predetermined power set point isapplied to the second RF signal.
 6. The method of claim 1, wherein theparameter is gamma, or forward power, or reflected power, or a voltageand current probe measurement, or a complex impedance, wherein theparameter is sensed at an output of an RF generator.
 7. A methodcomprising: generating a first RF signal; sending the first RF signal toan impedance match network that is coupled to a plasma chamber; sensinga parameter indicating a change in an impedance of plasma within theplasma chamber, wherein the change in the impedance occurs when a stateof a second RF signal changes from one state to another; and pulsing thefirst RF signal from one state to another in response to sensing theparameter indicating the change in the impedance.
 8. The method of claim7, wherein the parameter is sensed at an output of an RF generator thatgenerates the second RF signal.
 9. The method of claim 7, wherein thesecond RF signal is a low-frequency RF signal and the first RF signal isa high-frequency RF signal, wherein the high-frequency is greater thanthe low-frequency.
 10. The method of claim 7, wherein the one state ofthe first RF signal is an on state and the other state of the first RFsignal is an off state, wherein the one state of the second RF signal isan on state and the other state of the second RF signal is an off state,wherein the pulse of the second RF signal is generated upon receiving acontrol signal from a computer.
 11. The method of claim 7, wherein thefirst RF signal is pulsed from the one state to the other state when apredetermined power set point is applied to the first RF signal, whereinthe first RF signal is pulsed from the other state to the one state whenanother predetermined power set point is applied to the first RF signal,wherein the parameter is sensed at an output of an RF generator.
 12. Amethod for providing a plurality of synchronized pulsing RF signals to aplasma processing chamber of a plasma processing system, comprising:pulsing a first RF signal, using a first RF generator, said first RFsignal provided to said plasma processing chamber to energize plasmatherein; detecting values of at least one parameter associated with saidplasma processing chamber that reflects whether said first RF signal ispulsed high or pulsed low; and pulsing a second RF signal, using asecond RF generator, responsive to said detecting said values of said atleast one parameter.
 13. The method of claim 12, wherein said at leastone parameter that reflects whether said first RF signal is pulsed highor pulsed low represents at least one of forward RF power and reflectedRF power.
 14. The method of claim 12, wherein said at least oneparameter that reflects whether said first RF signal is pulsed high orpulsed low represents gamma, said gamma representing a numerical indexindicating a degree of mismatch between reflected power and forwardpower of said second RF generator.
 15. The method of claim 12, furthercomprising receiving a trigger threshold value from a tool hostcomputer, said trigger threshold value to enable circuitry in a sensorsubsystem of said second RF generator to ascertain whether said first RFsignal is pulsed high or pulsed low.
 16. The method of claim 12, whereinsaid second RF signal comprises at least a high level and a low levelwhen pulsed, said high level and said low level governed by at least onevalue provided a tool host computer.
 17. The method of claim 12, whereinsaid second RF signal, when pulsed, comprises at least a high pulsevalue and a low pulse value, wherein said low pulse value is non-zero.18. The method of claim 12, wherein said at least one parameter thatreflects whether said first RF signal is pulsed high or pulsed lowrepresents values obtained from a VI probe or represents an outputimpedance of said second RF generator.
 19. The method of claim 12,further comprising a match subsystem coupled to outputs of said first RFgenerator and said second RF generator, wherein said at least oneparameter that reflects whether said first RF signal is pulsed high orpulsed low represents an impedance of an input of said match subsystem.20. The method of claim 12, wherein said second RF signal, when pulsed,pulses between a predefined high pulse value and a predefined low pulsevalue.